While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less.
The resist materials for use in photolithography using light of an excimer laser, especially ArF excimer laser having a wavelength of 193 nm, are, of course, required to have a high transparency to light of that wavelength. In addition, they are required to have an etching resistance sufficient to allow for film thickness reduction, a high sensitivity sufficient to eliminate any extra burden on the expensive optical material, and especially, a high resolution sufficient to form a precise micropattern. To meet these requirements, it is crucial to develop a base resin having a high transparency, rigidity and reactivity. None of the currently available polymers satisfy all of these requirements. Practically acceptable resist materials are not yet available.
Known high transparency resins include copolymers of acrylic or methacrylic acid derivatives and polymers containing in the backbone an alicyclic compound derived from a norbornene derivative. All these resins are unsatisfactory. For example, copolymers of acrylic or methacrylic acid derivatives are relatively easy to increase reactivity in that highly reactive monomers can be introduced and acid labile units can be increased as desired, but difficult to increase rigidity because of their backbone structure. On the other hand, the polymers containing an alicyclic compound in the backbone have rigidity within the acceptable range, but are less reactive with acid than poly(meth)acrylate because of their backbone structure, and difficult to increase reactivity because of the low freedom of polymerization. It is noted that the term “(meth)acrylate” is used to mean acrylate or methacrylate. Additionally, since the backbone is highly hydrophobic, these polymers are less adherent when applied to substrates. Therefore, some resist compositions which are formulated using these polymers as the base resin fail to withstand etching although they have satisfactory sensitivity and resolution. Some other resist compositions are highly resistant to etching, but have low sensitivity and low resolution below the practically acceptable level.
Both the (meth)acrylic and alicyclic backbone systems commonly have the problem of pattern disruption due to swelling of resist film. Resist compositions based on these systems have been designed so as to improve their resolution performance by increasing the difference in dissolution rate before and after exposure, and as a consequence, they eventually become highly hydrophobic. Highly hydrophobic resist compositions, when applied as a film and processed with a developer, can maintain the film tenaciously in unexposed regions and allow the film to be instantaneously dissolved in over-exposed regions, while relatively broad exposed regions therebetween allow penetration of the developer, but are kept undissolved, that is, swollen. At the very small pattern size for which ArF excimer laser is actually used, those resist compositions which allow adjacent pattern strips to be joined together and disrupted on account of swelling are rejected. While a finer pattern rule is being demanded, there is a need to have a resist material which is not only satisfactory in sensitivity, resolution, and etching resistance, but fully restrained from swelling.